1. Field of Invention
This invention relates to a public key cryptography technology, and more particularly relates to a system for allowing only an arbitrary member in a group to decrypt and write a signature by use of a group key which is allowed to be used by only the group member.
2. Description of Related Art
Cryptography system called as public key cryptography system is disclosed in U.S. Pat. No. 4,200,770. In the public key cryptography system, a public key is used for encrypting a plain text and a private key is used for decrypting a cryptography text to a plain text. The public key and the private key are different each other. The public key may be disclosed to the public literally to be known by the public. On the other hand, in the conventional cryptography system (referred alternatively to as private key cryptography system, symmetric key cryptography system, and conventional cryptography system), the same key has been used for encryption and decryption, and how to secure the confidentiality has been the most serious problem, however in this public key cryptography system, the confidentiality of an encryption key is needless. In the case that the number of persons who communicates encrypted documents is n, n×(n−1)÷2 keys are necessary for the conventional encryption-decryption common key system, but only n keys are necessary for the public key cryptography system, this is the advantage of the public key cryptography system. In addition, the public key cryptography system is advantageous in that the same frame can be used in writing the signature of each person, namely encryption processing by each person by use of the private key. For example, a cryptography communication member P having the private key A converts a communication document X by use of the private key A, and sends the converted document Y and the communication document X to another member Q, then the member Q converts the converted document Y by use of the public key B of the member P and can confirm that it is surely the document that is sent by the member P if the conversion result of Y is identical with X. As described herein above, the public key cryptography system is advantageous in several points, which the conventional cryptography system does not have.
Japanese Published Unexamined Patent Application No. Hei 7-297818 discloses the structure of allocation of the public key and the private key to a group. This system is based on the assumption that a group private key is embedded in a physical substance such as a card and a group member is carrying this card consistently. The management of the key is realized based on the physical substance that is the card separated from the everlasting existence namely individual by structuring the cryptography system of the above-mentioned private key and public key using the substance namely the card.
In the public key cryptography system, the everlasting existence such as an individual is set as an independent unit. Therefore, the public key cryptography system cannot function fully in the case that it is necessary to set, for example, a plurality of members other than an individual as a single unit. Further, the system in which a card as described herein above is used is disadvantageous in that a hardware namely a card must be used inevitably, a card itself must be managed, and the verification of a card holder is problematic when the card holder misses the card due to loss or theft, in other word, it is difficult to verify the card holder in due course.
For example, an organization such as a department, a section, and a unit in a company is a cooperative working unit, and also an organization which is independent from hierarchical organizations such as a task force consisting of a plurality of individuals is a cooperative working unit. The confidentiality should be maintained between the inside and the outside of a cooperative working unit, but on the other hand, the information should be common among individual members within the corporation. A cryptography system which allows an arbitrary member of the cooperative working unit to decrypt or write a signature on the common information is required.
Further, in some occasion, a member of the cooperative working unit is required to change, for example, to add or to delete, the information, therefore the cryptography system should be allowable for a member to operate easily for such changing. Further, to take a part of the officer, like a chief of personnel department in a company as in a cooperative working unit, it is required that specific and continuous confidentiality corresponding to the role should be maintained independently of a specific individual who takes a part of the role at some time point in the form which is acceptable of the change of the individual who takes a part of the role.
The present invention provides a cryptography system for solving the above-mentioned problem. It is an object of the present invention to provide a cryptography system in which a public key cryptography system can be used in group units, each of which is a group consisting of components of individuals and groups instead of individual units so that members who belong to a specific group can decrypt the cryptography.
Further it is another objective of the present invention to provide a signature system which enables an arbitrary member who belongs a specific group to write a signature and to verify that a signature on a signed document is the signature written by a member who belongs to the specific group.